Cancer stem cell characteristics of circulating tumor cells

To review the results from previous studies aiming at the molecular characterization of circulating tumor cells (CTC), to specifically address the role of epithelial-mesenchymal-transition (EMT) and stemness features of CTC in their contribution to tumor progression and to summarize potential interference of CTC with the efficacy of radiotherapy.

Detection of CTC has been reported for most epithelial tumors and has been associated with an increased risk of local and regional recurrence as well as the development of distant metastases. Given a causal relationship between the presence of CTC and tumor progression at the primary or distant sites, several distinct features have to be postulated for these cells: First, a change from an epithelial to a mesenchymal cell-like phenotype which should alleviate the disconnection of individual tumor cells from tight cell-to-cell junctions within the epithelial cell layer and endow single tumor cells with the capacity to migrate into blood vessels; secondly, the presence of stem-cell properties which contribute to the re-establishment of bulk tumor tissue at the primary or metastatic site upon tumor recurrence or distant progression, respectively. Indeed, EMT and stem-cell features were frequently observed in CTC and the phenotype of CTC was established as a stronger predictor of outcome than sole enumeration of CTC in a defined volume of blood. The exploitation of CTC above their use as prognostic marker is still a subject of many ongoing investigations as are the identification of suitable therapeutic targets for this small cell subpopulation.